A turbomachine stage has a cascade of rotating rotor blades and a cascade of guide vanes disposed adjacent to the rotor blade cascade on the upstream or downstream side. The blade and vane airfoils terminate in an airfoil platform at the root end. The tip end may also be provided with an airfoil platform, for example, in the form of a guide vane or rotor blade shroud.
An axial gap is formed between the guide vane cascade and the rotor blade cascade. When the rotor blade cascade rotates, pressure gradients are formed therein, the pressure gradients varying around the circumference and causing secondary flows. For example, a rotating cascade of turbine rotor blades may force working fluid into the axial gap on its pressure side and, conversely, draw working fluid from the gap on its suction side. As a result, a compensating flow is generated, which degrades the efficiency of the turbomachine.
A gas turbine having shroudless rotor blades is disclosed in EP 2 372 102 A2, which proposes that the radially inner platforms of guide vanes and rotor blades have a non-axisymmetric contour, in particular a radially and/or axially undulated contour. This publication gives no information about the size and/or positioning of crests and troughs. According to the Figures, troughs are formed near the leading and trailing edges of the guide vanes and rotor blades, respectively.